Participants will spend several days at a high-altitude (approximately 18,000 feet above sea-level) camp. From two weeks prior to departure from Albuquerque to high-altitude until the descent begins about ten days later, six of the participants will be taking oral pentoxifylline 400 mgs three times a day and six will be taking placebo. In Albuquerque and at the high-altitude camp, xylose tolerance tests will be done on all participants.